1. Field of the Invention
The present invention relates to a frequency mixer circuit, and more specifically to a frequency mixer circuit of a direct conversion type for use in a receiving circuit such as a radio pager.
2. Description of Related Art
In the prior art, this type of frequency mixer circuit has been used in a circuit such as a radio pager receiving circuit which is required to operate with a low voltage. The frequency mixer circuit can be divided into two circuit types, namely, a single-balanced type, which is more general in the prior art and which will be called hereinafter a "first prior art frequency mixer circuit", and a double-balanced type which will be called hereinafter a "second prior art frequency mixer circuit".
Referring to FIG. 1, there is shown a circuit diagram of a typical example of the first prior art frequency mixer circuit. This first prior art frequency mixer circuit includes a differential circuit 10 composed of a pair of transistors Q7 and Q8 having their collector connected to a power supply voltage VCC through load resistors RL3 and RL4, respectively, their base biased with a bias voltage VBB3, and their emitter common-connected to each other, and an emitter-grounded, current source transistor Q9 having a collector connected to the common-connected emitters of the transistors Q7 and Q8, and a base biased with a biais voltage VBB4.
Now, operation of the first prior art frequency mixer circuit will be described with reference to FIG. 1.
A high frequency signal V1, which is constituted of a local oscillation signal, is supplied to only the base of the transistor Q7 in the differential circuit 10, and on the other hand, another high frequency signal V2, which is constituted of a received signal, is supplied to the base of the transistor Q9. In addition, the collector of the transistors Q7 and Q8 are connected to output terminals T1 and T2, respectively, between which an output voltage VO appears. With this arrangement, assuming that respective frequencies of the high frequency signals V1 and V2 are f1 and f2, the output voltage VO includes a frequency sum component f1+f2 and a frequency difference component If .vertline.f1-f2.vertline., and therefore, the shown circuit operates as a frequency mixer.
However, since the high frequency signal V1 constituted of the local oscillation signal, is a large-amplitude signal for alternately causing, the transistors Q7 and Q8 to flow the collector current, but since the high frequency signal V1 is supplied to only the base of the transistor Q7 in the form of an unbalanced signal, in the operation of the circuit there occurs a period in which only the transistor Q7 in the differential circuit is put in a saturated condition or in a condition near to the saturated condition. In this connection, such a phenomenon is known that, in the saturated condition of a turned-on transistor, electric charges are accumulated between the base and the emitter of the turned-on transistor, and therefore, when it is attempted to cut off the transistor, the transistor is not instantly turned off by action of the accumulated electric charges. With the action of electric charges accumulated between the base and the emitter of the transistor Q7 during a turned-on period of the transistor Q7 because of the same action as the above mentioned phenomenon, the turned-on period of the transistor Q7 becomes longer than that of the transistor Q8, and therefore, an averaged collector current of the transistor Q7 becomes different from that of the transistor Q8. As a result, a difference occurs between an averaged voltage on the output terminal T1 and an averaged voltage on the output terminal T2. Namely, an offset voltage occurs between the output terminals T1 and T2.
In a front-end circuit such as the radio pager operating with a low voltage, a high conversion gain and a low noise factor (NF) can be obtained when an output is balanced. However, in the first prior art frequency mixer circuit as mentioned above, since the offset voltage occurs between the output terminals because of the above mentioned reason, it is necessary to take a countermeasure, for example, to capacitively couple between succeeding stages, in order to supply a balanced output to a next stage.
Here, considering to use the first prior art frequency mixer circuit in the direct conversion type radio pager receiver, since a modulated wave is directly converted into a base band signal of 4.5 kHz, the capacitive coupling to the next stage requires a capacitor having a large capacitance, which, however, is very difficult to be formed on an integrated circuit. If this large capacitance capacitor is provided externally to the integrated circuit, the number of external terminals and the number of externally mounted circuit parts are increased. This is a hindrance in miniaturizing and in reducing the number of parts.
Referring to FIG. 2, there is shown a circuit diagram of a typical example of the second prior art frequency mixer circuit of the double-balanced type, which is disclosed by John F. Wilson et al, "A Single-Chip VHF and UHF Receiver for Radio Paging", IEEE Journal of Solid-State Circuits, Vol.SC-26, No.12, pp.1944-1950, Dec. 1991, and by Y. MORI, "Pager Receiver Design Technology", Chapter 3: Direct-Conversion system, pp.95-116, published from K. K. Torikeppusu, 1994, both of which disclose an elementary technology for the pager receiver, and the disclosure of which is incorporated by reference it their entirety into this application.
This second prior art frequency mixer circuit includes a first differential circuit 12 composed of a pair of transistors Q1 and Q2 having their emitter common-connected to each other, a second differential circuit 14 composed of a pair of transistors Q3 and Q4 having their emitter common-connected to each other, and a pair of current source transistors Q10 and Q11 having their collector connected to the conmmon-connected emitters of the transistors Q1 and Q2 of the differential circuit 12 and the common-connected emitters of the transistors Q3 and Q4 of the differential circuit 14, respectively. A base of each of the current source transistors Q10 and Q11 is connected to receive a bias voltage VBB5, and an emitter of the current source transistors Q10 and Q11 are connected to ground through resistors RE1 and RE2, respectively,
A collector of the transistors Q1 and Q3 are connected to each other, and also connected to an output terminal T1 and througah a load resistor RL1 to the power supply voltage VCC. A collector of the transistors Q2 and Q4 are connected to each other, and also connected to an output terminal T2 and through a load resistor RL2 to the power supply voltage VCC. A base of the transistors Q1 and Q4 are connected to each other, and a base of the transistors Q2 and Q3 are connected to each other.
A high frequency signal V1, which is constituted of a local oscillation signal, is supplied between the base of the transistor Q1 and the base of the transistor Q2, and on the other hand, another high frequency signal V2, which is constituted of a received signal, is supplied between the emitter of the transistor Q10 and the emitter of the transistor Q11. An output voltage VO is outputted between the terminals T1 and T2.
Similarly to the first prior art frequency mixer circuit, the output voltage VO of this second prior art frequency mixer circuit includes a frequency sum component f1+f2 and a frequency difference component .vertline.f1-f2.vertline. of the frequencies f1 and f2 of the high frequency signals V1 and V2, and therefore, the circuit operates as a frequency mixer.
Thus, a circuit portion operating to respond to the high frequency signal V1 is constituted to take a double-balanced structure by using the two differential circuits 12 and 14, and the high frequency signal V1 is inputted in the form of a balanced signal. Therefore, the above mentioned alternate turning-on of the differential pair transistors causes no difference in the averaged collector currents. As a result, an averaged voltage on the output terminal T1 and an averaged voltage on the output terminal T2 become the same constant level, and therefore, no offset voltage occurs between the output terminals T1 and T2. Accordingly, the circuit can be directly coupled to a next stage of circuit.
In the second prior art frequency mixer circuit, however, since each of the high frequency signals V1 and V2 is inputted in the form of a balanced signal, the circuit needs four input external terminals for receiving the two high frequency signals V1 and V2 when the frequency mixer circuit is implemented in a bipolar transistor integrated circuit. In addition, in order to convert the conventional unbalanced signal into a balanced signal, a circuit part such as a transformer is additionally required. Therefore, similarly to the first prior art frequency mixer circuit, the number of external terminals and the number of externally mounted circuit parts are increased. This is a hindrance in miniaturizing and in reducing the number of parts.
Furthermore, if the second prior art frequency mixer circuit is operated with a low power supply voltage as in the radio pager receiver, since the emitter resistors RE1 and RE2 are inserted between the emitter of the transistors Q10 and Q11 and the ground, there occurs a voltage loss corresponding to a voltage drop across the resistors RE1 and RE2. This is also a disadvantage of the second prior art frequency mixer circuit.